The stomach is an important organ of the digestive system that plays a role in storing food or drink for several hours during which the food or drink is rendered acidic by the action of secreted gastric acid and thereby prevented from spoiling while it is digested by digestive enzymes.
Gastric cancer occurs at a frequency of approximately 50 to 60 per 100,000 population in Japan and is more common in males than in females with a male-to-female ratio of 1 to 2:1. Also, gastric cancer kills approximately 50,000 people a year, which account for approximately 17% of the number of deaths caused by all cancer types, and was thus ranked No. 1 in the site-specific cancer mortality until the early 1990s after World War II. Gastric cancer is now ranked No. 2 following lung cancer, as the number of patients has been declining every year. Still, many patients suffer from this disease. On a world scale, gastric cancer affects many patients in Asian countries, such as Japan, South Korea, and China, and in South America. Examples of risk factors of gastric cancer can generally include smoking, high-salt diets, and infection with Helicobacter pylori. 
Endoscopic therapy, surgery, chemotherapy, radiation therapy, and the like are known as the treatment of gastric cancer and performed in consideration of disease stage, tumor size/depth, the degree of metastasis, etc. The course of treatment is determined on the basis of the “Gastric Cancer Treatment Guidelines” prepared by the Japanese Gastric Cancer Association in 2004. Early gastric cancer can be completely resected endoscopically or surgically and also has a low rate of recurrence. Advanced gastric cancer, on the other hand, recurs in many cases, even after extirpation of lesions, due to micrometastasis that has not been found at the time of operation. Gastric cancer provides a relatively favorable prognosis when found at an early stage, and typically, 90% or more cases are completely healed. However, the outcome of large tumor or after metastasis has a poor 5-year survival rate of approximately 70%. Hence, its early detection is important.
Unfortunately, most cases of gastric cancer have no symptoms at an early stage and do not produce recognizable subjective symptoms until the cancer is advanced. Thus, gastric cancer is difficult to early detect based on subjective symptoms. With the progression of gastric cancer, loose stool, black stool, nausea, gastric distress, and the like are found as subjective symptoms, and fatigability, fever, weight loss, anemia, and the like are found as systemic symptoms. In a more advanced stage, a lump is felt in the abdominal region as tumor increases in size. Even after appearance of such subjective symptoms, patients tend to often neglect them, and in many cases, already advanced cancer is detected by radiography or the like during medical examination. Hence, it is important to develop an examination method for highly sensitively and accurately detecting gastric cancer at an early stage.
Gastric cancer can be examined by a diagnostic imaging method such as ultrasonography, CT scan, angiography, or radiography. The diagnostic imaging method is useful in detecting small tumor in early gastric cancer, but is less than efficient when directed to many human test subjects, for example, in medical check-up, and disadvantageously requires relatively high cost for diagnosis.
With technical progress on genomics or proteomics in recent years, various novel tumor marker candidates have been being found as a result of research in the cancer field (e.g., Patent Literatures 1 and 2). Since a highly sensitive marker in blood specific for particular cancer probably allows relatively inexpensive high-throughput examination or diagnosis, its development is strongly demanded. Examples of methods for searching for a marker include a method involving comparing gene expression or the amount of proteins or cell metabolites or the like between cancer cells and non-cancerous cells, and a method involving measuring the amount of mRNA, proteins, or metabolites or the like contained in the body fluids of cancer patients and patients without cancer. For example, CEA, BFP, NCC-ST-439, CA72-4, and CA19-9 are known as tumor markers for gastric cancer currently used in clinical setting. Also, marker candidates have been found histologically, such as pepsinogen C (Non Patent Literature 1), hnRNP A2/B1 (Non Patent Literature 2), NSP3, transgelin, prohibitin, HSP27, protein disulfide isomerase A3, and GRP58 (Non Patent Literature 3). Unfortunately, these markers and marker candidates have poor specificity and/or detection sensitivity, or efficient methods for detecting them from biological samples have not yet been established. Thus, use thereof is limited to a narrow range of purposes such as posttreatment follow-up. Hence, a gastric cancer marker having higher specificity and detection sensitivity is desired.